Cathode-follower oscillator



Feb. 17 1959 J.'C. SAMUELS CATHODE-FOLLOWER OSCILLATOR Filed Sept. 27, 1955 IN V EN TOR.

JO/l/V C; Sam/2.52 BY J /l emf/2 'CATHODE=FOLLOWER-10SCILLATOR John Clifton Samuels, WestIJafayette,1nd,, assignor to International. Telephone and Telegraph, Corporation Applicationrseptember 227 .1955; Not 536,863:

4 claims. (Cl. 215.0%36) This invention relates. to oscillators and "is particularly directed) to osci1lators of'th'e cathode-follower type.

Inth'e. Proceedings offthe Institute of" Radio Engineers vol." 33; pages 8 1L855." of" December, 1945; Kurt Schlesinger treated cathode-follower circuits generally, and disclosed an oscillator in which the required" phase shift for sustained oscillations .was accomplished in a network in the cathode circuit ofan amplifier tube. The Schlesinger: network; looks; likegfl; Golpitts tanlc circuit wtih the; cathodeand the grid. and anqdeatapp ed to the middle and ends of. the tank. Such-an oscillator lacks ,a;

resistance conneetionito the grid, and lacks-famplitudeandz waveshape controlot: the generated wave. That; is smcethereis no resistance element common to the;

vcathode? and. anode circuits; there; is no, self regulation of; therspacey current :of theaamplifiertube. Such-a cincuitav iscvulnerable to power. supp1y=volta ge1changesand: isnot subject to, easy; changesyof frequency.

The: object of this, invention is;- an improved cathode.-

follower oseillator which is relatively; stable in frequency;

in responses tog-changes iii-power, supply voltages has: good; waveform; and is. easily. variedin; frequency.

The; objectsv of this: invention. are; attained {by two types;

of cathode-follower oscillators; (1;); A cat-hode follower; oscillaton in which the necessary, regeneration. is accom,

plished; by. a; bridge network, 2); at cathode-follower phasershif osoillaton lXl- WhlCh; the necess-aryregeneratiomis accomplished by; an resistance-capacitance ladder network: of; gainwgreater th n; unity... hQbIidgC network; oscillator; comprises a fourrsideda bridge network con; nected between the grid: and'cathode of; a, cathode fo 1. A311; inductive element is connected across; one; diagonal of the bridge: and; capacitive cle lower amplifier tube.

ments; are connected to; twoopposite sides of the bridge androsistiveelements. are connected. in the remaining: two oppositmsidesmftho:bridge. The grid: and cathode; or;

the amplifier-.1 are, respectively, COllllQGtQCli across the other diagonal of the bridge.

The, ahovesmentionedz and th r fea ur s and bje of.: this:inven.tion and. he: mann r. f ttaining. hemiwillw become: more. apparent and. the; inventionv itselfi will be;

bestunderstood: by reference,- to .the; following; description of: an: embqdiment of the: invention taken in. conjunction with the accompany ng. draw gs, wh n:

Fig. 1 is; a circuit diagram: of. a. convention l, kI QW cathode-follower amplifier;-

gcl is a redrawn. circu tz iag zamsof the1 amplifier of Fig. 3 is; a: circuit diagram of the cathode-iollower 0s} l 65 Fig. 4 1s a circuit diagram, the same-asthc CIIQUlt;

cillator of. thi inv ntion;

resolved into the circuit of" Fig. 1 inwhich the cathode 2,874,290 Patented. bi sist nces..R ndtRaare qnneswdin srie w i hi het p at l a e,- ou qe.Ebt-., RIB s... m.n9a.w hths. f

rcuit... he at to e eak' e istance R'ajssonae tstli een. he rid; n. 1. he- PP sl fflt thq s r sis q E; he giny mr ont m otthe ampl fie for li ean nera icn. t low r uenc wher a stthetampl fic icn act tt mbe. 3; is th cathode r sist r r isah tub plate resistanc e isith tinputsignal, n

an st e utpi ts gn Itwillybe noted that the gainof such a cathode-fol;

lower-amplifier is inevitably less than unity. Intypical operation of the commercially obtainable tubes such as the triogle Sf/"18, where the amplification factor is 27 and-the plate resistance is 5.0 K -ohms-an d the cathode-- resistor is 10 K ohms, the gain of the tube as'a-cathod efollower-amplifier is 0:95.

The circuit of Fig. 1 may-be modified as shown in Fig. 2where R and R may be omitted'sincesthese two components do notyenter-into the'considerations to follow. It can beassumed'thatthe gain of; the cathodefollower is essentially unchanged by the described' modification even though. there isacertain loss in gain due to the loading effect of condensers C Cg, and resistance he mpedan e, oo in v ntq he pu erminals. 1 and Zis':

Z12? l' FiA'WC R when no grid current flows and operation is at-medium frequency.

It can be seen that values of; C C R and R can be. so chosen that he resis imno tiq i f Zi nega iv t follows that an appropriate element connected between terminals 1- and 2 will result in the functioning of: the modified circuit as'an oscillator. Since the reactive portion of Z is capacitive, the. appropriate element in thiscase would bean inductor connected between" terminals 1 and 2. The final configuration of the oscillator, com-v prising this invention, thenfitakes the form shown in Fig. 3.

In Fig. 3, cathode resistor R is common to the anode circuit and the grid circuit of; the grid control amplifier 10. The voltage of the anode source 11 is the rated voltage for the particular amplifier 10 chosen, The re: sistance R and the inductance L comprise the. galyanic grid circuit of the oscillator. Condenser C is connected between the cathode of the amplifier tube and the junction of resistor R and inductance L. Condenser C is connected in series with resistance R across inductance L. In the circuit of Fig. 3, no grid return resistor or grid 'bias'resi stor, aside from the resistances of the phase. shifting network, is required. Bias and amplitude control are provided byresistor R and capacity 6 Res.

sistor R can be so adjusted that nearly Class-A operation work eanproperly be drawn as a four-sided bridgetwit-h inductance L across one diagonal of the bridge and with the grid and cathode connected across the other diagonal of the bridge. The values ofC R and C R, maybe so chosen that the proper phase and amplitude relation for Barkhausen conditions exist for sustained oscillations. Under these conditions, the output wave of the oscillator, appearing across the resistor R which forms one arm of the bridge network, is applied to the inductance-capacitance, L-section consisting of L and C The values of L and C are such that, at the frequency of the output signal they are near resonance on the high side of the resonance curve. As a consequence, the portion of the output voltage appearing across L is amplified (enough to overcome losses of the next section) and shifted in phase (leading). This signal is then applied to the resistance-capacitance network comprising the other two arms of the bridge network. In passing through this network the phase lead resulting from passing through the LC section is cancelled by the phase lag of this section. The signal across C is thus in phase with the output signal e passing through the RC section is suehthat the signal arriving at the grid is equal to that which existed at the grid at the start of the cycle around the bridge network. Thus, the amplifier is capable of maintaining self-oscillations.

The slight non-linearities in the grid causes rectification; thus enabling resistor R and capacitor C to develop grid bias and thereby stabilize the oscillator. Resistor R and inductor L serve as a grid leak eliminating the buildup of static charge on the grid.

The output frequency is stable with respect to supply voltage variations. The reason for this can be understood from a consideration of the following equation for determining the frequency of self-oscillation Where H(s) is the transfer function of the bridge network,

s=parameter of the Laplace transformation, V ==plate resistance of the tube =amplification factor of tube R kZi R k Z i R =cathode load resistor and Z,=input impedance to the bridge network.

Clearly Z does not depend on the tube characteristics.

or the supply voltage. It is V and to a lesser extent a which are sensitive to supply voltage variations. But when the bridge circuit parameters and R are properly proportioned and generally 7 E121 if is large. Hence the frequency equation becomes approximately The voltage loss resulting from the signal C =250 micromicrofarads. The triode was of the 5718 type and the gain of the amplifier thus connected is .80:

With these constants, it is {found that the frequency of oscillation is fairl sensitive to all parameters except the coil resistance, and that the best control over frequency could be obtained by varying C The frequency range was found to be from low audio to middle radio frequencies. Frequency stability during temperature and humidity variations is primarily determined by'the dependence of the resistor, capacitor, and inductive elements upon these factors.

If a resistor R is inserted in the plate lead of the cathode-follower oscillator of this invention, a variable frequency balanced voltage source may be developed by adjustment of R until the voltage across R equals the output voltage e The insertion of R in the plate circuit will not materially affect the operation of the oscillator if the plate supply voltage is adjusted to give the same plate voltage as existed before the insertion of R Referring to the expression for gain A tabulated above, it is found that the frequency of oscillation is affected by R,, only when R is relatively of large value. Because of the factor #E-lthe affect of R,, on frequency is considerably reduced.

The other cathode follower oscillator attaining the objectives of this invention is shown in Fig. 5 wherein a phase-shift network, of the RC type, is coupled between the cathode resistor and the grid of the amplifier. The network is of the ladder type with series condensers C C et cetera, and shunt resistors R R R etc. The input terminals of the phase-shift network are connected directly across the cathode resistors R and one output terminal of the network is connected directly to the control grid of amplifier 10. In this oscillator, oscillations are maintained by regenerative feedback from the cathode to the grid. This is achieved by phase shifting the signal from the cathode through three (or any appropriate number) RC filters, but taking the output to the grid across the capacitors instead of across the last resistor of the RC ladder. By doing this a voltage gain is obtained from the RC network to overcome the loss through the cathode-follower amplifier. Nevertheless, the signal undergoes no phase shift in traversing the ladder. Thus it arrives in phase with the signal on the grid producing the regenerative effect. This circuit is unusual in that it uses no inductors, no plate feedback and a single tube; yet it is capable of self-oscillation. The essential fact is the over-unity gain RC network for phase-shifting.- This network coupling between the cathode resistor and the-control grid results in an oscillator of greater stability than can be expected from the standard phase-shift oscillator where the phase-shift network is between the anode and the control grid.

The cathode-follower oscillators of this invention at of the feedback type and are capable of adjustment over a frequency band from low audio frequency range to the low or middle radio frequency range without extensive complications. The frequency of oscillation can be estimated from the linear theory of oscillation, is very stable with respect to supply voltage variations, and is reasonably stable with cathode loading.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. A cathode-follower oscillator comprising a foursided bridge network with an inductive elementconnected across one diagonal of the bridge, capacitive elements in opposite two sides of the bridge, and resistive elements in the remaining opposite two sides, a grid-controlled amplifier with the grid and cathode of the amplifier connected across the other diagonal of said bridge.

2. A cathode-follower oscillatorcomprising an amplitier tube with a cathode, a control grid and an anode, an inductance device, a first series-connected capacitance and a resistance connected, respectively, between opposite ends of said inductance and said cathode, a second seriesconnected capacitance and a resistance connected, respectively, between said opposite ends of they inductance, the juncture of said second series-connected capacitance and resistance being connected to said grid, the relative values of the reactive and resistive elements being such that the grid receives oscillatory potentials in phase with the oscillatory' potential of said cathode.

3. A cathode-follower oscillator comprising an amplifier tube with a cathode, a grid, and an anode; a foursided bridge network, an inductance connected across one diagonal of the bridge, phase-shifting series reactive elements constituting two adjacent sides of the bridge and connected across the inductance, second phase-shifting series reactive elements constituting the other two adjacent sides of the bridge and connected across the inductance, and intermediate points of the first and second mentioned series reactive elements being connected, respectively, to:

References Cited in the file of this patent UNITED STATES PATENTS Bowes Dec. 26, 1933 Holbrook et a1. Oct. 30, 1956 OTHER REFERENCES An R-C Circuit Giving Over-Unity Gain, by Longmire, in TeleTech, pages 40, 41, 112, April 1947.

Free. I. R. 13., vol. 39, No. 7, July 1951, pp. 833-835, Synthesis of lassive RC Networks With Gains Greater Than Unity, Epstein. I v

R-C Networks With Over-Unity Gain, by Bacon, in Wireless Engineer, pages 20-23, January 1953. 

